Change based on age of oil or mileage?

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My wife drives our 2008 ML 320 CDI about 5,000 per year, and the suggested oil change mileage is 7,500 miles. Should it be changed semi-annually, annually, or just ignore time and go by mileage?
 
The Mercedes oil life monitor will tell you, based on either mileage or time. And it will tell you to change at one year if the mileage isn't exceeded first.
 
CDI is a diesel engine. I'm still learning but I'm under the impression that the high sulfur diesel creates more acid that TBN is there to battle with to remain alkaline. I would change it and get a used oil analysis with TBN and TAN if you want to know the true answer. If it were my car I would change it the next time it is most convenient for me, even if that is a month or two from now.
 
Originally Posted By: 901Memphis
There's no way oxidation is a factor until at least 2 years if the mileage isn't reached.


Is that true for pretty much any off the shelf oil? We don't know what is in his car.
 
Originally Posted By: kevhal777
I just got results back from a UOA done with 3000 miles and 2 years on the oil, no makeup oil added. I have a 30 quart pan so 3000 miles is nothing. The results were excellent. TBN was 10.3, all the element ppm's were well under the universal averages, insolubles was 0.1, no water, antifreeze, trace fuel. Blackstone said to run it out to 7000 miles next time. At my use rate, that would be at least 4 years. I really wonder now why there is so much emphasis on changing every 6 mos. or year if you haven't reached a mileage-based interval. The results of this test don't give me any reason to change it at 2 years, which is already double the time interval everyone says is critical.


Considering the highest TBN of 11-12(?) for HDEO, time did nothing to the oil. If the Beretta lives long enough to run the 15K I plan for the 0w40 that's in it, I'm thinking of taking a sample. The oil should be there for close to 5 years at that point since I drive 4 other vehicles. Although I pray to God I won't drive that bucket for that long.
 
Originally Posted By: Shark
Originally Posted By: 901Memphis
There's no way oxidation is a factor until at least 2 years if the mileage isn't reached.


Is that true for pretty much any off the shelf oil? We don't know what is in his car.


If the oil meets the manufacturer's recommendation and you are staying within the manufacturers mileage recommendation I'd say it's true for any of those oils, obviously some are more stout than others.

Oxidation is occurring at all times but I have yet to see a low mileage lengthy time UOA in a healthy engine that showed oxidation thickening or high oxidation in general, but I gave seen a few 4 year uoas with low mileage and just beginning to show thickening due to oxidation
 
That is all very interesting and maybe a very good point. We all use to change our dino oils based on advertised advice and common habits. Now more and more people are using synthetics and are told they can go longer now. I think oil companies want you to change your oil so you will buy more but also on the bottle it may say xx miles or one year to save their butts too just like oil filter companies. Amsoil says that but also says to shorten it if more severe or a UOA. There are too many variables to know for sure unless one sends in a sample. That's the only way that I know the oil is okay for yet another year without any problems, whether it's too much fuel, antifreeze, or your silicon levels are rising due to an airfilter needing to be changed. Why even a catalytic converter causing too much back pressure can effect your oil. If one wants to push the envelope in time or mileage, it just needs to be monitored to be safe. But if everything stays ok, I wonder just how long an oil can stay in a crankcase because 1 1/2 years is the longest I have run.
 
Originally Posted By: 901Memphis
Originally Posted By: Shark
Originally Posted By: 901Memphis
There's no way oxidation is a factor until at least 2 years if the mileage isn't reached.


Is that true for pretty much any off the shelf oil? We don't know what is in his car.


If the oil meets the manufacturer's recommendation and you are staying within the manufacturers mileage recommendation I'd say it's true for any of those oils, obviously some are more stout than others.

Oxidation is occurring at all times but I have yet to see a low mileage lengthy time UOA in a healthy engine that showed oxidation thickening or high oxidation in general, but I gave seen a few 4 year uoas with low mileage and just beginning to show thickening due to oxidation



This is very good information, and timely for me. I was debating changing PYB in a Wrangler that is at 13 months but only 1000 miles. I will run it atleast another year.
 
I would do it annually.

Running a hardly used diesel for 18months with the same oil is asking for trouble in the long term.
 
Spoke with customer service of three major fully integrated oil corps that market motor oil with the same question, mileage or time since my vintage vehicles go for months and sometimes years without starting the engine.
All customer reps had the same recommendation.

Unopened shelf life is four years for multigrade.

Maximum life in the engine is 12 months for multigrade.

Even if the engine is not started or driven under severe conditions.
If driving habits are repeatable, like 20% short trips, 40% Highway stop & go and remaining Highway
or
Short trip home to school and back home, 100% of the time is considered severe duty. This typical for high school students or folks that have really short job commutes.
Had a job were I drove only two miles to train station and parked the vehicle to hop on a commuter train then another two mile trip back home in the evening. This routine is considered severe duty.
Then get a sample of the used oil and have it analyzed for wear and remaining usable life. Costs about $40
Lab will advise how often to change oil based in driving habits.
On short trips the oil does not get sufficiently hot to boil off the moisture absorbed into the oil from the atmosphere and will thicken forming sludge.
Rainy regions like Oregon have more issues with water contamination than hot desert regions.

Mono grade oil like straight 30wt will last much longer because there are no viscosity modifiers included in the base oil.
The viscosity modifiers decompose with time.
 
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